When I use my OneDrive and invite specific people, their email addresses show up in the comments, while in SharePoint it says "guest user" as it does with the anonymous link. Does anyone know how I can fix that? I'm transferring some shared documents over to SharePoint, but we aren't ready to add people to sites yet (very much a noobie to this, but need to get these shared documents out of my personal folder!).
If you share documents or folders via individual links by Share > Link settings > Specific people, the external user will not be added to the organization directory and will have access to the shared objects via the sharing links only.
Sharing a list/library or a site to external users is a different process. The external users will have to accept the invitation sent via Email and then sign in/register with a Microsoft or Organization account in order to be added to the organization directory, or as you put, to become a “guest user”.
You can remove the guest users from this tenant via Microsoft 365 admin center > Guest users > Delete a user. It does not have influence on the links you have already shared in SharePoint lists or libraries.
Reference: External sharing overview.